Retina CSS is generated with background-size and correctly scaled size

Changes the way retina CSS is output, so the dimensions are specified independent of the file's resolution and the background image resized with `background-size`. See: [Retina displays, high-res background images](http://stackoverflow.com/questions/16154494/retina-displays-high-res-background-images) and [5 Things I Learned Designing For High-Resolution Retina Displays](http://www.leemunroe.com/designing-for-high-resolution-retina-displays/) for examples.

Without this change, retina sprites aren't displayed properly and appear at full pixel size in the browser.